1. Field of the Invention
This invention relates to curable compositions that can be stored for extended periods of time in the absence of molecular oxygen and contain a platinum-group metal based hydrosilation catalyst that is at least partially inhibited under anaerobic conditions. Curing of the compositions is achieved by exposing them to molecular oxygen, which activates the catalyst and initiates curing of the composition under ambient conditions or with heating.
2. Background Information
Curable compositions containing polyorganosiloxanes or organosiloxane-modified organic polymers yield crosslinked products ranging from flowable gels to elastomers to resins and find use in a wide variety of applications. These compositions can be classified based on the reaction used to cure the compositions and the conditions required to cure the compositions. One class of compositions cure by a hydrosilation reaction that is catalyzed by a metal from the platinum group of the periodic table or a compound of such a metal. These metals include platinum, rhodium and palladium.
The advantages of curing using a hydrosilation reaction include not requiring the presence of moisture or excessively high temperatures to initiate the curing reaction. The curing reaction occurs when all of the reactants, namely an organic polymer or polyorganosiloxane containing at least two alkenyl radicals per molecule and a compound containing at least two silicon bonded hydrogen atoms per molecule, are combined with the hydrosilation catalyst under ambient conditions. The obvious disadvantage of this type of curable composition is its poor storage stability. If it is not convenient to prepare the composition just prior to use, one method for overcoming this disadvantage is to prepare a two part composition by packaging the hydrosilation catalyst and the compound containing the silicon-bonded hydrogen atoms in separate containers.
One-part curable compositions with storage stabilities ranging from days to months depending upon the temperature can be prepared using known platinum catalyst inhibitors such as acetylenic compounds and amines or by encapsulating the hydrosilation catalyst in a material that is melted or fragmented to release the catalyst when it is desired to cure the composition. Both of these methods have disadvantages associated with them, including increased cost, relative high curing temperature and/or extended curing times.
The effect of anaerobic conditions, i.e. the absence of molecular oxygen, on the platinum-catalyzed reaction between organohydrogensilanes and alkenes or non-polymeric organosilicon compounds containing ethylenic unsaturation is reported in the patent and journal literature. U.S. Pat. No. 4,578,497, which issued to Onpochenko and Sabourin on Mar. 25, 1986 reports that oxygen enhanced the rate of the platinum catalyzed hydrosilation of 1-alkenes with mono-, di- and trialkylsilanes to yield the corresponding tetraalkylsilanes. Examples of the types of platinum catalysts exhibiting this effect include a platinum catalyst having the basicity substantially equal to or less than that provided by a platinum-containing catalyst having a triphenylphosphine ligand or a heterogeneous platinum-containing catalyst. Specific catalysts exhibiting enhanced activity in the presence of oxygen included chloroplatinic acid, the Wilkinson catalyst [RhCi(PPh.sub.3).sub.3 ], and carbon-supported platinum metal catalysts.
L. Lewis and N. Lewis in the journal Chemistry of Materials (1989, 1, 106-14) report that the reaction between platinic chloride and dimethylethoxysilane does not occur in the absence of molecular oxygen.
An article by L. Lewis [J. Am. Chem. Soc., 112, 16, 5998 (1990)] reports that formation of a catalytically active colloid by the reaction of a platinum compound with a stoichiometric excess of a silane or disiloxane containing silicon-bonded hydrogen atoms did not occur in the absence of molecular oxygen. The author considers the colloid to be the initiating species for hydrosilation reactions. Other hydrosilation reactions using this type of colloidal catalyst prepared using platinum and rhodium compounds are described by L. Lewis and R. Uriate in the journal "Organometallics" Vol. 9, pages 621-625 (1990).
An article by J. F. Harod and A. J. Chalk in a text entitled "Organic Synthesis via Metal Carbonyls" (I. Wender and P. Pino, Eds., Wiley (New York), 1977, Vol. 2, pp. 682-683) reports that complexes of the type [(olefin)PtCl.sub.2 ].sub.2 prepared under anaerobic conditions are totally inactive as hydrosilation catalysts with triethoxysilane, even though addition of the triethoxysilane resulted in decolorization of the initially yellow platinum/olefin complex. Catalytic activity was restored when the olefin complex was exposed to oxygen, even when the hydrosilation reaction was carried out under anaerobic conditions. The anaerobic inhibition was not observed using trichlorosilane and triethylsilane.
Rate reductions under anaerobic conditions of hydrosilation reactions involving various silanes containing silicon bonded hydrogen atoms with various olefins and vinylsilanes using rhodium and iridium catalysts, including rhodium/phosphine complexes, relative to the rate of the identical reactions in air are reported by H. Dickers et al. in the Journal of the Chemical Society, Dalton Transactions, Vol. 2, (1980) pp. 308-313.
These studies on the effect of molecular oxygen on hydrosilation reactions catalyzed by platinum group metals and compounds of these metals used conventional platinum group metal hydrosilation catalysts. The reactants in the hydrosilation reaction did not include polyorganosiloxanes capable of forming gels, elastomers, resins and similar high molecular weight crosslinked materials.
Organosiloxane compositions that cure upon heating by a platinum-catalyzed hydrosilation reaction and contain a "conventional" platinum catalyst and a phosphine or phosphite as the platinum catalyst inhibitor are claimed in U.S. Pat. No. 3,188,300 that issued to Chalk on Jun. 8, 1965. Organosiloxane compositions containing these compounds are alleged to be stable for periods up to 24 hours "or longer" at 25.degree. C. There is no teaching in this patent concerning the effect of or any advantage associated with preparing and storing the ingredients of the curable composition and the composition itself in the absence of oxygen.
Organosiloxane compositions that are stable in the absence of molecular oxygen but cure to yield elastomers or other useful materials in the presence of oxygen by a variety of reactions are known in the art. For example, U.S Pat. No. 3,249,580, which issued to Beck on May 3, 1966 describes anaerobically stable compositions containing copolymers prepared by reacting a halogen-terminated polysilane containing at least two silicon-bonded hydrogen atoms per molecule with a member of a class of silanol-containing organosilicon compounds that includes polyorganosiloxanes.
U.S. Pat. No. 4,526,954, which issued to Williams on Jul. 2, 1988 describes linear and cyclic organosiloxane polymers that cure in the presence of oxygen but are stable under anaerobic conditions. The polymers are characterized by the presence of at least one 1,3-pentadienyl or 1,4-pentadienyl radical per molecule. The group containing this radical is bonded to silicon through a silicon-carbon bond. This patent also describes curable compositions containing the novel polymers in addition to fillers and curing catalysts.
Published European Patent Application No. 511,882-A1, published on Nov. 4, 1992, describes polyorganosiloxanes containing cyclopentadiene or cyclopentadienyl groups bonded to silicon directly or through a hydrocarbylene radical. The polyorganosiloxanes cure in the presence of moisture or oxygen.
One objective of this invention is to provide anaerobically inhibited compositions that cure in the presence of molecular oxygen to yield elastomers, foams, resins and other useful crosslinked materials. A second objective is to provide a method for preparing these curable compositions using novel hydrosilation catalysts that are at least partially inactivated in the absence of oxygen.